This application claims the benefit of U.S. Provisional Application No. 60/164,859 filed Nov. 12, 1999. should appear as the first sentence of the description.
The present invention relates to the use of a novel enzyme and its encoding gene for transformation. More specifically, the invention relates to the use of a gene encoding an enzyme with acyl-CoA : diacylglycerol acyltransferase activity. This gene expressed alone in transgenic organisms will increase the total amount of oil (i.e. triacylglycerols) that is produced.
In oil crops like rape, sunflower, oilpalm etc., the oil (i.e. triacylglycerols) is the most valuable product of the seeds or fruits and other compounds such as starch, protein and fiber is regarded as by-products with less value. Enhancing the quantity of oil per weight basis at the expense of other compounds in oil crops would therefore increase the value of the crop. If enzymes regulating the allocation of reduced carbon into the production of oil can be upregulated by overexpression, the cells will accumulate more oil at the expense of other products. This approach could not only be used to increase the oil content in already high oil producing organisms such as oil crops, they could also lead to significant oil production in moderate or low oil containing crops such as soy, oat, maize, potato, sugar beats, and turnips as well as in microorganisms.
Development in genetic engineering technologies combined with greater understanding of the biosynthesis of triacylglycerols now makes it possible to transfer genes coding for key enzymes involved in the synthesis of triacylglycerols from a wild plant species or organisms of other kingdoms into domesticated oilseed crops. In this way, triacylglycerols can be produced in high purity and quantities at moderate costs.
It is known that the biosynthesis of triacylglycerols (TAG) in fat-accumulating tissues in animals (Bell and Coleman, 1983) and plants (Cao and Huang, 1986, Martin and Wilson 1983) as well as the accumulation of oil in microbial organisms such as bacteria (Ekundayo and Packler, 1994), yeast and other fungi (Ratledge 1989) can be catalyzed by acyl-CoA : diacylglycerol acyltransferases (DAGATs), enzymes that transfer an acyl-group from acyl-CoA to diacylglycerol, thus forming TAG.
During the past few years genes coding for DAGATs, have been identified in animals (Cases et al., 1998), plants (Hobbs et al., 1999; Lardizabal et al., 2000) and in microbes (Lardizabal et al., 1999). These DAGATs belong to two unrelated protein families.
The first type of DAGAT that was characterized, DAGAT A, has so far been found only in animals (Cases et al., 1998) and plants (Hobbs et al. 1999). These genes show sequence similarities to genes encoding acyl-CoA : cholesterol acyltransferase (ACAT). The mouse DAGAT A has 20% amino acid sequence identity to the mouse ACAT (Cases et al., 1998). However, DAGATs A from plants and animals are more similar to each other than to ACAT. Thus, the mouse DAGAT A has 38% amino acid sequence identity to the Arabidopsis thaliana DAGAT A (Hobbs et al., 1999). It is also approximately 80% identical to the human ACAT like protein ARGP1, which was suggested to be involved in TAG synthesis (Oelkers et al., 1998), indicating that ARGP1 is a DAGAT A.
The yeast S. cerevisiae contain 2 genes with sequence similarity to ACAT, ARE1 and ARE2 (Yang et al., 1996). The encoded proteins have approximately 24% overall amino acid sequence identity to the mouse ACAT and 15% identity to the DAGAT A from mouse. It should be noted that they are both more similar to each other (45% amino acid sequence identity) than to either ACATs or DAGATs from higher eukaryotes. It is not possible to classify them as putative ACATs or DAGATs based on sequence similarities alone, since their evolutionary distances from both groups of higher eukaryotic enzymes are similar. However, experiments have shown that both Are1 and Are2 are ACATs, which together are responsible for all of the sterol ester synthesis that occurs in yeast (Yang et al., 1996; Yu et al., 1996). The possible involvement of Are1 and Are2 in the synthesis of TAG has also been studied (Yang et al., 1996; Yu et al., 1996). From these studies, it was concluded that Are1 and Are2 are not involved in TAG synthesis. Thus, these is no prior art to show that Are1 is a TAG synthesizing enzyme, nor can it be concluded, on the basis of homologies to ACAT like sequences already published, that Are1 is a DAGAT (Lassner and Ruezinsky, 1999).
The second family of DAGAT enzymes, the DAGAT B family, is unrelated to any other known proteins. These enzymes show no sequence homology to the mouse and plant ACAT like DAGAT A proteins (Lardizabal et al., 2000) or to any other known proteins.
DAGAT A and B are not the only enzymes that contribute to TAG biosynthesis. TAG can also be synthesized by an acyl-CoA independent reaction. Thus, the newly discovered enzyme phospholipid : diacylglycerol acyltransferase (PDAT) catalyses the formation of TAG by transferring an acryl group from the sn-2 position of a phospholipid to DAG (Dahlqvist et al., 1999; St{dot over (a)}hl, 1999).
This invention describes the identification of a gene encoding an enzyme that is partly responsible for TAG accumulation in yeast.
In a first embodiment, this invention is directed to the TAG synthesising enzyme comprising an amino acid sequence as set forth in SEQ ID NO 2 or a functional fragment, derivative, variant, ortologue or isoenzyme thereof.
The present invention further includes the nucleotide sequence as set forth in SEQ ID NO 1, as well as portions of the genomic sequence, the cDNA sequence, allelic variants, synthetic variants and mutants thereof. This includes sequences that codes for variants of the polypeptide set forth in the sequence listing including biologically active triacylglycerol synthesising enzymes as well as sequences that are to be used as probes, vectors for transformation or cloning intermediates.
Another aspect of the present invention relates to hose polypeptides, which have at least 60% identity to SEQ ID NO 2. Preferred embodiments are polynucleotides that encode polypeptides with diacrylglycerol acyltransferase activity.
In a different aspect, this invention relates to the use of these nucleotide sequences in recombinant DNA constructs to direct the transcription and translation of the diacylglycerol acyltransferase sequence of the present invention in a host organisms or progeny thereof, including oil seeds, yeast and other fungi, as well as other oil accumulating organisms. Cells and organisms containing the diacylglycerol acyltransferase as a result of the production of the acyltransferase encoding sequence are also included within the scope of the invention.
Of particular interest is the expression of the nucleotide sequences of the present invention from transcription initiation regions that are preferentially expressed in plant seed tissues. It is contemplated that the gene sequence may be synthesized, especially when there is interest to provide plant-preferred codons.
In a different embodiment, this invention also relates to methods of using a DNA sequence encoding a said protein of the present invention for increasing the oil-content within the cells of different organisms.
Further, the invention makes possible a process for the production of triacylglycerol, which comprises growing transgenic cells or organisms under conditions whereby any of the nucleotide sequences discussed above are expressed in order to produce an enzyme in these cells with the ability to transfer a fatty acid from acyl-CoA to diacylglycerol, thus forming triacylglycerol.
Moreover, triacylglycerols produced by the aforementioned process are included in the scope of the present invention.
The present invention can be essentially characterized by the following aspects:
1. Use of a nucleic acid sequence encoding an enzyme catalysing the transfer of a fatty acid from acyl-CoA to diacylglycerol for the production of triacylglycerol (TAG) by genetic transformation of an oil-producing organism with said sequence in order to be expressed in this organism and result in an active enzyme in order to increase the oil content of the organism.
The nucleic acid sequence is derived from the sequence shown in SEQ ID NO. 1, from the Saccharomyces cerevisiae ARE1 gene (genomic clone or cDNA), or from a nucleic acid sequence or cDNA that contain nucleotide sequences coding for a protein with an amino acid sequence that is 60% or more identical to the amino acid sequence as presented in SEQ. ID. NO. 2.
2. Transgenic organisms comprising, in their genome or on a plasmid, a nucleic acid sequence according to the above, transferred by recombinant DNA technology. The transgenic organisms are selected from the group consisting of fungi, plants and animals. Preferably the transgenic organisms agricultural plants and preferably said nucleotide sequence is expressed under the control of a storage organ specific promoter. Alternatively, the nucleotide, the nucleotide sequence is expressed under the control of a seed-specific promoter.
3. Oils from organism according to aspect 2.
4. A protein encoded by a DNA molecule according to SEQ ID NO. 1 or a functional (enzymatically active) fragment thereof. Alternatively, the protein produced in an organism as specified in aspect 2, which has the amino acid sequence set forth in
SEQ ID NO. 2 or an amino acid sequence with at least 60% homology to said amino acid sequence. Preferably the protein is isolated from Saccharomyces cerevisiae. 
5. Use of a protein as specified in aspect 4 in the production of triacylglycerols.
6. Triacylglycerols according to aspect 5.